Japanese Patent Application Laid-Open No. 10-215366 discloses a compressed-image-data extraction method in which, each time a boundary between encoded data items is detected, the beginning-point positional information and the ending-point positional information of the encoded data for the corresponding image per processing unit are stored, and the boundary position is identified based on the stored positional information items.
For the purpose of extracting encoded data for a desired partial image from compressed image data, the technique disclosed in Japanese Patent Application Laid-Open No. 10-215366 separately stores and utilizes positional information with which the boundary of the partial image can be identified.